1. Field of the Invention
This invention relates to means of determining the load on a cable supported by a freely suspended multiple sheave assembly.
2. Description of Prior Art
For such activities as lowering wire cables to which sensors and instruments are attached, as in hydrographic work from ships, a freely suspended sheave is employed.
U.S. Pat. No. 4,492,363 discloses a sheave assembly having five pulleys and a pair of retainer pulleys positioned immediately adjacent the cable on either side of a center pulley so as to maintain the cable with a constant bend around the center pulley, regardless of the angle between the two ends of the cable extending from the sheave assembly. By virtue of this arrangement a line monitor is operatively coupled to the center pulley for measuring the tension, speed and footage of the cable. More specifically, magnets adjacent to the center pulley are used in the measurement of line footage and speed by measuring the speed and direction of the rotation of the center pulley and a strain gauge, located under the suspension eye-bolt, is used in the measurement of line tension. With a known constant bend of the cable around the center pulley, the measurements of the strain gauge can be directly translated into line tension.
As shown in the brochure entitled Counterbalanced Block/Wire Metering Systems, manufactured by General Oceanics, in another commercially available arrangement, a single pulley is used with a lever-like cable-following arm. As the angle of the cable (as defined by the two straight cable portions extending downward from the pulley) increases the arm is moved upward. A sensor (such as a hub-mounted potentiometer having a resistance that varies as a function of the angle of the arm) measures the angle of the cable. Additionally, a strain gauge, located under the suspension eye-bolt, is operatively coupled to measure the downward force of the cable on the pulley. From the downward force measured by the strain gauge and the angle of the line measured by the line follower, the load and therefore tension on the cable can then be calculated. In the prior art, this is typically done by digitizing the analog signals from the two sensors and computing therefrom the load. Such commercially available devices are shown in the previously mentioned brochure.
This load information is used to keep the cable tension within a specified range by controlling a winch to which one end of the cable is operatively coupled. In this manner, the cable is kept from being too tight or too loose.